Sabermetric Research

Phil Birnbaum

Thursday, April 30, 2015

Math class is tough

"Picture this: You're at the supermarket trying to find the best deal on AAA batteries for your flashlight, so you check the price labels beneath each pack. Sounds pretty straightforward, right? But how can you tell which pack is cheaper when one is priced per battery and one is priced per 100?"

-- Consumer Reports, June 2015, calling for government regulation of "unit pricing" labels

Saturday, April 18, 2015

Making the NHL draft lottery more fun

The NHL draft lottery is to be held today in Toronto. Indeed, it might have already happened; I can't find any reference to what time the lottery actually takes place. The NHL will be announcing the results just before 8:00 pm tonight.(Post-lottery update: someone on Twitter said the lottery took place immediately before the announcement. Which makes sense; the shorter wait time minimizes the chance of a leak. Also: the Oilers won, but you probably know that by now.)The way it works is actually kind of boring, and I was thinking about ways to make it more interesting ... so you could televise it, and it would hold viewers' interest.(Post-lottery update: OK, they did televise a medium-sized big deal for the reveal. But, since the winner was already known, it was more frustrating than suspenseful. This post is about a way to make it legitimately exciting while it's still in progress, before the final result is known.)Let me start by describing how the lottery works now. You can skip this part if you're already familiar with it.------The lottery is for the first draft choice only. The 14 teams that missed the playoffs are eligible. Whoever wins jumps to the number one pick, and the remaining 13 teams keep their relative ordering. The lower a team's position in the standings, the higher the probability it wins the lottery. The NHL set the probabilities like this: 1. Buffalo Sabres 20.0% 2. Arizona Coyotes 13.5% 3. Edmonton Oilers 11.5% 4. Toronto Maple Leafs 9.5% 5. Carolina Hurricanes 8.5% 6. New Jersey Devils 7.5% 7. Philadelphia Flyers 6.5% 8. Columbus Blue Jackets 6.0% 9. San Jose Sharks 5.0%10. Colorado Avalanche 3.5%11. Florida Panthers 3.0%12. Dallas Stars 2.5%13. Los Angeles Kings 2.0%14. Boston Bruins 1.0%It's kind of interesting how they manage to get those probabilities in practice.The NHL will put fourteen balls in a hopper, numbered 1 to 14. It will then randomly draw four of those balls.There are exactly 1,001 combinations of 4 balls out of 14 -- that is, 1,001 distinct "lottery tickets". The 1001st ticket -- the combination "11, 12, 13, 14" -- is designated a "draw again." The other 1,000 tickets are assigned to the teams in numbers corresponding to their probabilities. So, the Sabres get 200 tickets, the Hurricanes get 85 tickets, and so on. (The tickets are assigned randomly -- here's the NHL's .pdf file listing all 1,001 combinations, and which go to which team.)It's just coincidence that the number of teams is the same as the number of balls. The choice of 14 balls is to get a number of tickets that's really close to a round number, to make the tickets divide easily.------So this works like a standard lottery, like Powerball or whatever: there's just one drawing, and you have the winner. That's kind of boring ... but it works for the NHL, which isn't interested in televising the lottery live.But it occurred to me ... if you DID want to make it interesting, how would you do it?Well, I figured ... you could structure it like a race. Put a bunch of balls in the machine, each with a single team's logo. Draw one ball, and that team gets a point. Put the ball back, and repeat. The first team to get to 10 points, wins.You can't have the same number of balls for every team, because you want them all to have different odds of winning. So you need fourteen different quantities. The smallest sum of fourteen different positive integers is 105 (1 + 2 + 3 ... + 14). That particular case won't work: you want the Bruins to still have a 1 percent chance of winning, but, with only 1 ball to Buffalo's 14, it'll be much, much less than that.What combinations work? I experimented a bit, and wrote a simulation, and I came up with a set of 746 balls that seems to give the desired result. The fourteen quantities go from 70 balls (Buffalo), down to 39 (Boston). In 500,000 runs of my simulation, Buffalo won 20.4 percent of the time, and Boston 1.1 percent. Here are the full numbers. First, the number of balls; second, the percentage of lotteries won; and, third, the percentage the NHL wants.-------------------------------------------------- result target-------------------------------------------------- 1. Buffalo Sabres 70 balls 20.4% 20.0% 2. Arizona Coyotes 63 balls 13.5% 13.5% 3. Edmonton Oilers 61 balls 11.5% 11.5% 4. Toronto Maple Leafs 58 balls 9.3% 9.5% 5. Carolina Hurricanes 57 balls 8.5% 8.5% 6. New Jersey Devils 56 balls 7.3% 7.5% 7. Philadelphia Flyers 54 balls 6.5% 6.5% 8. Columbus Blue Jackets 53 balls 5.9% 6.0% 9. San Jose Sharks 51 balls 4.7% 5.0%10. Colorado Avalanche 49 balls 3.7% 3.5%11. Florida Panthers 47 balls 3.2% 3.0%12. Dallas Stars 45 balls 2.5% 2.5%13. Los Angeles Kings 43 balls 1.9% 2.0%14. Boston Bruins 39 balls 1.1% 1.0%--------------------------------------------------The probabilities are all pretty close. They're not perfect, but they're probably good enough. In other words, the NHL could probably live with awarding the Bruins a 1.1% chance instead of a 1% chance.If you did the lottery this way, would it be more fun? I think it would. You'd be watching a race to 10 points. It would have a plot, and you could see who's winning, and the odds would change every ball. Every team would have something to cheer about, because they'd probably all have at least a few balls drawn. The ball ratio between first and last is only around 1.8 (70/39), so for every 9 points the Sabres got, the Bruins would get 5. The average number of simulated balls it took to find a winner was 72.4. If you draw one ball every 30 seconds ... along with filler and commercials, that's a one-hour show. Of course, it could go long, or short. The minimum is 10; the maximum is 127 (after a fourteen-way tie for 9). But I suspect the distribution is tight enough around 72 that it would be reasonably manageable.Another thing too, is ... every team would have a reasonable chance of being close, and an underdog will almost always challenge. Here's how often each team would finish with 7 or more points (including times they won): 1. Buffalo Sabres 53.3 percent 2. Arizona Coyotes 43.4 3. Edmonton Oilers 40.2 4. Toronto Maple Leafs 35.7 5. Carolina Hurricanes 33.9 6. New Jersey Devils 32.4 7. Philadelphia Flyers 29.5 8. Columbus Blue Jackets 27.7 9. San Jose Sharks 25.2 10. Colorado Avalanche 22.1 11. Florida Panthers 19.5 12. Dallas Stars 16.7 13. Los Angeles Kings 14.5 14. Boston Bruins 10.6 And here's the average score, by final position after it's over:10.0 Winner 8.0 Second 7.3 Third 6.6 ... 6.1 5.6 5.2 4.8 4.3 3.9 3.5 3.0 2.4 1.6That's actually closer than it looks, because you don't know which team the bottom one will be. Also, just before the winning ball is drawn the first-place team would have been at 9.0, which means, at that time, the second-place team would, on average, have been only one point back. ------The problem is ... it still takes 746 balls to make the odds work out that closely. That's a lot of balls to have to put in the machine. Of course, that's just what I found by trial and error; you might be able to do better. Or, you could use a smaller number of balls, and accept a probability distribution that's different from the NHL's, but still reasonable.Or, you could add a twist. You could give every ball a different number of points. Maybe the Sabres' balls are worth from 5 points down to 1, and the Bruins' balls are only 3 down to 1, and the first team to 20 points wins. I don't think it would be that hard to find some combination that works. That's kind of a math nerd thing. I'd bet you can find a system that comes as close as I got with less than 100 balls, and I bet you'd be able to get to it pretty quick by trial and error. At least, the NHL could, if it wanted to.

Saturday, April 11, 2015

MLB forecasters are more conservative this year

Every April, sabermetricians, bookies and sportswriters issue their won-lost predictions for each of the 30 teams in MLB. And, every year, some of them are overconfident, and essentially wind up trying to predict coin flips.As I've written before, there's a mathematical "speedoflight" limit to how much of a team's record can be predicted. That's the part that's determined by player talent. Any spread that's wider than the spread of talent must be just random luck, and, by definition, unpredictable.Based on the historical record, we can estimate that the spread of team talent in MLB is somewhere around an SD of 9 games. Not all of that talent can be predicted beforehand, because some of it hasn't happened yet -- trades, injuries, players unexpectedly blossoming or declining, and so on. My estimate is that if you were the most omnicient baseball insider in the universe, maybe you could predict an SD of 8 games.Last year, many pundits exceeded that "speed of light" limit anyway. I thought that there would be fewer this year, that the 2015 forecasts would project a narrower range of team outcomes. That's because last year's standings were particularly tight, and there's been talk about how we may be entering a new era of parity.And that did happen, to some extent.I'll show you the 2015s and the 2014s together for easy comparison. A blank space is a forecast I don't have for that year. (For 2015, I think Jonah Keri and the ESPN website predicted only the order of finish, and not the actual W-L record.) Like last year, I've included the "speed of light" limits, the naive "last year regressed to the mean" forecast, and the "every team will finish .500" forecast. Links are for 2015 ... for 2014, see last year's post. 2015 2014-------------------------------------------------- 9.32 11.50 Sports Illustrated 9.07 8.76 Jeff Passan (Yahoo!) 9.00 9.00 Speed of Light (theoretical est.) 8.79 Bruce Bukiet 8.53 Jonah Keri (Grantland) 8.51 Sports Illustrated (runs/10) 8.00 8.00 Speed of Light (practical est.) 7.79 ESPN website 7.92 7.78 Mike Oz (Yahoo!) 6.99 Chris Cwik (Yahoo!) 6.38 6.38 Naive previous year method (est.) 6.34 9.23 Mark Townsend (Yahoo!) 6.10 6.90 Tim Brown (Yahoo!) 6.03 7.16 Vegas Betting Line (Bovada) 5.46 5.55 Fangraphs 4.93 8.72 ESPN The Magazine 0.00 0.00 Predict 81-81 for all teams--------------------------------------------------Of those who predicted both seasons, only two out of eight increased the spread of their forecasts from last year. And those two, Jeff Passan and Mike Oz, increased only a little bit. On the other hand, some of the other forecasters see *dramatically* more equality in team talent. Yahoo's Mark Townsend dropped from 9.23 to a very reasonable 6.34. And ESPN dropped from one of the highest spreads, to the lowest -- by far -- at 4.93. Which is strange, because ESPN's words are so much more optimistic than their numbers. about the Washington Nationals, they write,

"It's the Nationals' time."

"They're here to stay."

"Anything less than an NL East crown will qualify as a big disappointment."

But their W-L prediction for the Nationals, whom they projected higher than any other team? A modest 91-71, only ten games above .500.------In any case ... I wonder how much of the differences between 2014 and 2015 are due to (a) new methodologies, (b) the same methodologies reflecting a real change in team parity, and (c) just a gut reaction to the 2014 standings having been tighter than normal.My guess is that it's mostly (b). I'd bet on Bovada's forecasts being the most accurate of the group. If that's true, then maybe teams really *are* tighter in talent than last year, by around 1 win of SD. Which is, roughly, in line with the rest of the forecasts.I guess we'll know more next year.

Wednesday, April 08, 2015

How much has parity increased in MLB?

The MLB standings were tighter than usual in 2014. No team won, or lost, more than 98 games. That's only happened a couple of times in baseball history.You can measure the spread in the standings by calculating the standard deviation (SD) of team wins. Normally, it's around 11. Two years ago, it was 12.0. Last year, it was down substantially, to 9.4.Historically, 9.4 is not an unprecedented low. In 1984, the SD was 9.0; that's the most parity of any season since the sixties. More recently, the 2007 season came in at 9.3, with a team range of 96 wins to 96 losses.But this time, people are noticing. A couple of weeks ago, Ben Lindbergh showed that this year's preseason forecasts have been more conservative than usual, suggesting that the pundits think last year's compressed standings reflect increased parity of talent. They've also noted another anomaly: in 2014, payroll seemed to be less important than usual in predicting team success. These days, the rich teams don't seem to be spending as much, and the others seem to be spending a little more.So, have we actually entered into an era of higher parity, where we should learn to expect tighter playoff races, more competitive balance, and fewer 100-win and 100-loss teams? My best guess is ... maybe just a little bit. I don't think the instant, single-season drop from 12 games to 9.4 games could possibly be the result of real changes. I think it was mostly luck. -----Here's the usual statistical theory. You can break down the observed spread in the standings into talent and luck, like this: SD(observed)^2 = SD(talent)^2 + SD(luck)^2Statistical theory tells us that SD(luck) equals 6.4 games, for a 162-game season. With SD(observed) equal to 12.0 for 2013, and 9.4 for 2014, we can solve the equation twice, and get2013: 10.2 games estimated SD of talent 2014: 7.0 games estimated SD of talentThat's a huge one-season drop, from 10.2 to 7.0 ... too big, I think, to really happen in a single offseason. Being generous, suppose that between the 2013 and 2014 seasons, teams changed a third of their personnel. That's a very large amount turnover. Would even that be enough to cause the drop?Nope. At least, not if that one-third of redistributed "talent wealth" was spread equally among teams. In that case, the SD of the "new" one-third of talent would be zero. But the SD of the remaining two-thirds of team talent would be 8.3 (the 2013 figure of 10.2, multiplied by the square root of 2/3).That 8.3 is still higher than our 7.0 estimate for 2014! So, for the SD of talent to drop that much, we'd need the one-third of talent to be redistributed, not equally, but preferentially to the bad teams. Is that plausible? To how large an extent would that need to happen?We have a situation like this: 2014 talent = original two thirds of 2013 talent + new one third of 2013 talent + redistribution favoring the worse teamsStatistical theory says the relationship between the SDs is this:SD(2014 talent) squared = SD(2013 talent teams kept)^2 +SD(2013 talent teams gained)^2 + SD(2013 talent teams kept) * SD(2013 talent teams gained) * correlation between kept and gained * 2It's the same equation as before, but with an extra term (shown in bold). That term shows up because we're assuming a non-zero correlation between talent kept and talent gained -- that the more "talent kept," the less your "talent gained". When we just did "talent" and "luck", we were assuming there was no correlation, so we didn't need that extra part. (We could have left it in, but it would have worked out to zero anyway.)The equation is easy to fill in. We saw that SD(2014 talent) was estimated at 9.4. We saw that SD(talent teams kept) was 8.3. And we can estimate that SD(talent teams gained) is 12.0 times the square root of 1/3, which is 5.9.If you solve, you get Correlation between kept and gained = -0.57That's a very strong correlation we need, in order for this to work out. The -0.57 means that, on average, if a team's "kept" players were, say, 5th best in MLB (that is, 10 teams above average), its "gained" players must have been 9th worst in MLB (5.7 teams below average). That's not just the good teams getting worse by signing players that aren't as good as the above-average players they lost -- it's the good teams signing players that are legitimately mediocre. And, vice-versa. At -0.56, the worst teams in baseball would have had to have replaced one-third of their lineup, and those new players would have to have been collectively as good as those typically found on a 90-win team.Did that actually happen? It's possible ... but it's something that you'd easily be able to have seen at the time. I think we can say that nobody noticed -- going into last season, it didn't seem like any of the mainstream projections were more conservative than normal. (Well, with the exception of FanGraphs. Maybe they saw some of this actually happen? Or maybe they just used a conservative methodology.)One thing that WAS noticed before 2014 is that the 51-111 Houston Astros had improved substantially. So that's at least something.And, for what it's worth: the probability of randomly getting a correlation coefficient as extreme as 0.57, in either direction, is 0.001 -- that is, one in a thousand. On that basis, I think we can reject the hypothesis that team talent grew tighter just randomly.(Technical note: all these calculations have assumed that every team lost *exactly* one-third of its talent, and that those one-thirds were kept whole and distributed to other teams. If you were to use more realistic assumptions, the chances would improve a little bit. I'm not going to bother, because, as we'll see, there are other possibilities that are more likely anyway.)------So, if it isn't the case that the spread in talent narrowed ... what else could it be? Here's one possibility: instead of SD(talent) dropping in 2014, SD(luck) dropped. We were holding binomial luck constant at 6.4 games, but that's just the average. It varies randomly from season to season, perhaps substantially. It's even possible -- though only infinitesimally likely -- that, in 2014, every team played exactly to its talent, and SD(luck) was actually zero!Except that ... again, that wouldn't have been enough. Even with zero luck, and talent 10.3, we would have observed an SD of 10.3. But we didn't. We observed only 9.4. So, maybe we have another "poor get richer" story, where, in 2014, the bad teams happened to have good luck, and the good teams happened to have bad luck.We can check that, in part, by looking at the 2014 Pythagorean projections. Did the bad teams beat Pythagoras more than the good teams did?Not really. Well, there is one obvious case -- Oakland. The 2014 A's were the best team in MLB in run differential, but won only 88 games instead of 99 because of 11 games of bad "cluster luck". Eleven games is unusually large. But, the rest of the top half of MLB had a combined eighteen games of *good* luck, which seems like it would roughly cancel things out.Still ... Pythagoras is only a part of overall luck, so there's still lots of opportunity for the "good teams having bad luck" to have manifested itself. -----Let's do what we did before, and see what the correlation would have to be between talent and luck, in order to get the SD down to 9.4. The relationship, again, is:SD(2014 standings)^2 = SD(2014 talent)^2 + SD(2014 luck)^2 + SD(2014 talent) * SD(2014 luck) * correlation between 2014 talent and 2014 luck * 2 Leaving SD(2014 talent) at the 2013 estimate of 10.2, and leaving SD(2014 luck) at 6.4, we getCorrelation between 2014 talent and luck = -0.39The chance of a correlation that big (either direction) happening just by random luck is 0.033 -- about 1 in 30. That seems like a big enough chance that it's plausible that's what actually happened. Sure, 1 in 30 seems low, and is statistically significantly unlikely in the classical "1 in 20" sense. But that doesn't matter. We're being Bayesian here. We know something unlikely happened, and so the reason it happened is probably also something unlikely. And the 1/30 estimate for "bad teams randomly got lucky" is a lot more plausible than the 1/1000 estimate for "bad teams randomly got good talent." It might also be more plausible than "bad teams deliberately got good talent," considering that nobody noticed any unusual changes in talent at the time.------Having got this far, I have to backtrack and point out that these odds and correlations are actually too extreme. We've been assuming that all the unusualness happened after 2013 -- either in the offseason, or in the 2014 season. But 2013 might have also been lucky/unlucky itself, in the opposite direction.Actually, it probably was. As I said, the historical SD of actual team wins is around 11. In the 2013 season, it was 12. We would have done better by comparing the "too equal" 2014 to the historical norm, rather than to a "too unequal" 2013. For instance, we toyed with the idea that there was less luck than normal in 2014. Maybe there was also more luck than normal in 2013. Instead of 6.4 both years, what if SD(luck) had actually been 8.4 in 2013, and 4.4 in 2014?In that case, our estimates would beSD(2013 talent) = 8.6SD(2014 talent) = 7.6That would be just a change of 1.0 wins in competitive balance, much more plausible than our previous estimate of a 3.2 win swing (10.2 to 7.0).------Still: no matter which of all these assumptions and calculations you decide you like, it seems like most of the difference must be luck. It might be luck in terms of the bad teams winding up with the good players for random reasons, or it might be that 2013 had the good teams having good luck, or it might be that 2014 had the good teams having bad luck.Whichever kind of luck it is, you should expect a bounceback to historical norms -- a regression to the mean -- in 2015. The only way you can argue for 2015 being like 2014, is if you think the entire move from historical norms was due to changes in the distribution of talent between teams, due to economic forces rather than temporary random ones. But, again, if that's the case, show us! Personnel changes between 2013 and 2014 are public information. If they did favor the bad teams, show us the evidence, with estimates. I mean that seriously ... I haven't checked at all, and it's possible that it's obvious, in retrospect, that something real was going on.------Here's one piece of evidence that might be relevant -- betting odds. In 2014, the SD of Bovada's "over/under" team predictions was 7.16. This year, it's only 6.03.*(* Bovada's talent spread is tighter than what we expect the true distribution to be, because some of team talent is as yet unpredictable -- injuries, trades, prospects, etc.)Some of that might be a reaction to bettor expectations, but probably not much. I'm comfortable assuming that Bovada thinks the talent distribution has compressed by around one win.*Maybe, then, we should expect a talent SD of 8.0 wins, rather than the historical norm of 9.0. That's more reasonable than expecting the 2013 value of 10.2, or the 2014 value of 7.0.

If the SD of talent is 8, and the SD of luck is 6.4 as usual, that means we should expect the SD of this year's standings to be 10.2. That seems reasonable. ------Anyway, this is all kind of confusing. Let me try to summarize everything more understandably.------The distribution of team wins was much tighter in 2014 than in 2013. As I see it, there are six different factors that could have contributed to the increase in standings parity:-- 1. Player movement from 2013 to 2014 brought better players to the worse teams (to a larger extent than normal), due to changes in the economics of MLB.-- 2. Player movement from 2013 to 2014 brought better players to the worse teams (to a larger extent than normal), due to "random" reasons -- for instance, prospects maturing and improving faster for the worse teams.-- 3. There was more randomness than usual in 2013, which caused us to overestimate disparities in team talent.-- 4. There was less randomness than usual in 2014, which caused us to underestimate disparities in team talent.-- 5. Randomness in 2013 favored the better teams, which caused us to overestimate disparities in team talent.-- 6. Randomness in 2014 favored the worse teams, which caused us to underestimate disparities in team talent.Of these six possibilities, only #1 would suggest that the increase in parity is real, and should be expected to repeat in 2015. #3 through to #6 suggest that 2013 was a random aberration, and would suggest that 2015 would be more like the historical norm (SD of 11 games) rather than like 2013 (SD of 12 games). Finally, #2 is a hybrid -- a one-time random "shock to the system," but with hangover effects into future seasons. If, for instance, the bad teams just happened to have great prospects arrive in 2014, those players will continue to perform well into 2015 and beyond. Eventually, the economics of the game will push everything back to equilibrium, but that won't happen immediately, so much of the 2014 increase in parity could remain.------Here's my "gut" breakdown of the contribution each of those six factors:25% -- #1, changes in talent for economic reasons 5% -- #2, random changes in talent10% -- #3, "too much" luck in 201320% -- #4, "too little" luck in 201410% -- #5, luck favoring the good teams in 201330% -- #6, luck favoring the bad teams in 2014Caveats: (1) This is just my gut; (2) the percentages don't have any actual meaning; and (3) I could easily be wrong.If you don't care about the reasons, just the bottom line, that breakdown won't mean anything to you. As I said, my gut for the bottom line is that it seems reasonable to expect 2015 to end with a standings SD of 10.2 wins ... based on the changes in the Vegas odds.But ... if there were an over/under on that 10.2, my gut would say to take the over. Even after all these arguments -- which I do think make sense -- I still have this nagging worry that I might just be getting fooled by randomness.